Effect of Fill Ratio

The experiment has been tested with fill ratios ranging from 25% to 99%. The working

fluid used in all the experiments is R134a. After running the experiment with fill ratios of 25-99%

relative to the total volume of the pipe while removing working fluid in increments of

28

not able to reach steady state at a heat input of 200 W. The maximum heat input at which each fill

ratio experiment was able to reach steady state is listed in Table 4. Based on these results, the

experiments with fill ratios of 70-75% can transfer the most heat.

Table 4: Maximum Heat Input at which Each Fill Ratio Experiment Can Reach Steady State with R134a as Working Fluid

Fill Ratio (% relative to total loop volume)

Maximum Heat Input at which Steady State is Reached

99% Not able to reach steady state at 200W heat input

95% 200W 90% 300W 85% 300W 80% 300W 75% 325W 70% 325W 65% 300W 60% 300W 55% 300W 50% 300W 45% 300W 40% 300W 35% 300W 30% 250W

25% Not able to reach steady state at 200W heat input

The pressure changes within the system in response to a heat input with varying fill ratios

corresponding to the previously mentioned experiments were studied. For the experiments

discussed below, the pressures at the TC and BC location were plotted. Each plot also contains the

saturation pressure corresponding to the temperature recorded at the given location. The pressure

results for the 95% fill ratio experiment are shown in Fig. 14. The 99% and 95% fill ratio

experiment are the only experiments where the system reached the fully filled condition and

became single phase. The fully filled condition is defined as the condition when the working fluid

within the loop expands to fill the entire volume, and volume expansion is limited. This was

29

indicates when volume expansion is limited, since any increase in temperature after the working

fluid expands to fill the pipe results in a significant increase in pressure. The 95% experiment

reached steady state at 200W. Then, the heat input was increased in increments of 10W, and

allowed to reach steady state, until 260W when the pressure began to rise steeply. At this point,

the heating element was shut off, as indicated by the “0W” label on Fig. 14.

Figure 14: Pressure Response to Heat Input at a. TC (T4), b. BC (T1) for 95% R134a Fill Ratio and Heat Inputs from 200-260W

It can be seen in Fig. 14a that the pressure, after about 8000 seconds, begins to rise above

the saturation pressure at the TC (T4) location. This indicates the working fluid is in the

compressed liquid phase, rather than a saturated vapor or liquid-vapor mix. When the filly filled

state is reached, volume expansion is limited. Therefore, any additional increase in temperature is

accompanied by a rapid rise in pressure, as shown in Fig. 14. Figure 14b shows the saturation

pressure and system pressure at the BC location. The system pressure is always greater than the

saturation pressure, indicating the working fluid is always a compressed liquid at the BC location.

The 99% fill ratio experiment exhibited similar characteristics with a steep pressure rise occurring

30

ratio experiment reaches the fully filled condition earlier, as expected. The 99% and 95%

experiments operate as TPLTS until the system becomes fully filled and is no longer able to operate

due to the significant pressure rise.

The pressures at the flow/heater and BC locations are plotted for the 55% fill ratio

experiment in Fig. 15. As seen in Fig. 15, the saturation pressure is equal to or slightly greater than

the system pressure at the heater/flow location. This indicates the working fluid is vapor or liquid

vapor mix just after the heater. However, at the BC location, the system pressure is greater than

the saturation pressure, indicating that the working fluid at the BC of the loop is a liquid. With

lower fill ratios, including the 55% fill ratio, the NPCHP operates as a TPLTS.

Figure 15: Experimental Pressure Data of 55% R134a Fill Ratio with Heat Inputs of 200-300W Based on experimental data, the experiments with fill ratios of 95-99% reach single phase

since the system pressure is greater than the saturation pressure at all locations when a high enough

heat input is applied (250W for the 95% fill ratio experiment and 200W for the 99% experiment).

The experiments with fill ratios less than 95% are two-phase. The system pressures for these fill

ratio experiments at the heater location are less than or equal to the saturation pressure, indicating

vapor or liquid-vapor mix, and the system pressure at the BC location is greater than the saturation

31

the phase change is inhibited are the 95-99% experiments when sufficient heat is applied to the

system.

These experimental results agree with theoretical predictions based on volume expansion

and initial experimental conditions. The NPCHP is initially filled with a predetermined amount of

working fluid. As the temperature of the working fluid increases, it expands to fill the entire pipe

(if the initial fill ratio and heat input are high enough). According to Lee et al. (2010a, 2010b) the

working medium inside the NPCHP comes to a fully filled state under a certain heating condition.

In this state, the volume expansion and the phase change of the working medium in the pipe caused

by temperature rise is restrained. Table 5 shows the volume expansion coefficients corresponding

to the specific temperature and pressure of each experiment that was run ranging from 80-95% fill

ratios. The temperature change required to fill the entire pipe is calculated and added to the initial

temperature of the working fluid to determine the temperature the working fluid inside the

experiment must reach to fully fill the pipe.

Table 5: Volume Expansion Coefficients and Temperature Increase Required to Reach Fully- Filled State

Fill Ratio α (1/K) Tinitial (K) ΔT (K) Tfinal (K)

95% 3.670x10-3 292.71 14.34 307.05

85% 4.683x10-3 290.40 37.69 328.09

80% 4.589x10-3 289.22 54.48 343.70

The R134a within the loop should remain below 50oC (323K), as required by the safety

data sheet. This means that the fully filled state can be reached for fill ratios of 95% or greater,

which agrees with the experimental results obtained by comparing saturation and system pressures

32